It is a form of copper treatment that you use for parasites
Unless it is a mixture, it is not copper based. Formalin is the active ingredient in fomaldahyde, and it is used in SW
disease treatment as an anti parasitic. There is alot of debate about whether fish should be prophylactically treated for parasites and common bacterial infections. I am not a fan of prophylactic treatment, but I can see an LFSs point, they do spend alot of money on these fish and some parasites will kill the fish before it is noticed by staff that are busy doing their other duties. This is just another reason for a q-tank, so you can observe the fish and then treat as needed. Also the fish at the LFS
may be infected, but the treatment is only half done, and when you purchase the fish the signs are masked.
On another note I am an advocate of treating all WC
clowns with formalin for the prevention of brooklynella. This parasite is very fast acting and is possibly the leading cause of deaths in newly caught WC